Performance, none, the 3.8 V6 is an even firing engine, not needing a H/X pipe to equalize pressure from side to side.
Now then...sound. This is a debate that has gone on forever and will always continue. Some ppl say a H-pipe will give you a deeper sound than true duals. But...I don't notice this difference in sound. I have my exhaust (mach 1 takeoff's) run true duals, and I love the sound (just slightly louder and deeper than gt takeoff's). If it was me, I'd say go with the true duals.
Now that I actually read your post...I see that you wanna know the difference in sound between using a steeda adaptor and just welding the takeoff's straight up. The only big difference I can think of, is if you ever wanna take your exhaust off, the steeda adapter will work best. Running true duals, welding them right up (as I have my car done) is the cheapest way to do this by far....but whatever you really want to do man.
"I'm not driving too fast...just flying too low"
-03 SB Cobra vert- 2552 of 5082 Born 02/25/03
90 7up vert - new project-07 DSG Focus -DD